There’s a likely reason Google chose that specific value for their planned buyback: It’s what mathematicians call a “perfect number.”

Perfect numbers are whole numbers with a fun property – they are the sum of all their proper divisors, or the whole numbers that evenly divide into the original number, except itself. The smallest perfect number is 6: 6 is divisible by 1, 2, 3, and itself. But then take those first three divisors and add them up, and you get 1 + 2 + 3 = 6.

8,589,869,056 is such a number. It has a few too many divisors to write out here – 33 proper divisors in total – but the sum of its proper divisors does in fact come back up to itself.

Mersenne primes are interesting because it’s relatively computationally easy to check whether a candidate Mersenne prime is in fact prime. Because of that, most of the largest prime numbers yet discovered are Mersenne primes, including the largest currently known prime number, 277,232,917 – 1.

Only 50 Mersenne primes have been found so far, which means because of their correspondence, only 50 perfect numbers are known so far. That makes Google’s buyback choice quite a special number.